MCP606 Microchip Technology, MCP606 Datasheet - Page 12

• Manufacturer Part Number: MCP606
• Description: 2.5V to 5.5V Micropower CMOS Op Amps
• Manufacturer: Microchip Technology

MCP606/607/608/609
FIGURE 3-6:
3.4
The MCP608 is a single amplifier with a Chip Select
option. When CS is pulled high the supply current
drops to 50 nA (typ). In this state, the amplifier is put
into a high impedance state. By pulling CS low, the
amplifier is enabled. If the CS pin is left floating, the
amplifier will not operate properly. Figure 3-6 shows the
output voltage and supply current response to a CS
pulse.
3.5
In applications where low input bias current is critical,
PC board surface leakage effects and signal coupling
from trace to trace need to be taken into consideration.
3.5.1
Surface leakage across a PC board is a consequence
of differing DC voltages between two traces combined
with high humidity, dust or contamination on the board.
For instance, the typical resistance from PC board
trace to pad is approximately 10
conditions. If an adjacent trace is biased to 5V and the
input pin of the amplifier is biased at or near zero volts,
a 5pA leakage current will appear on the amplifier’s
input node. This type of PCB leakage is five times the
room temperature input bias current (1pA, typ) of the
MCP606/607/608/609 family of amplifiers.
The simplest technique that can be used to reduce the
effects of PC board leakage is to design a ring around
sensitive pins and traces. An example of this type of
layout is shown in Figure 3-7.
DS11177B-page 12
CS
Output
V
Current
DD
The Chip Select Option of the MCP608
Layout Considerations
SURFACE LEAKAGE
Supply
Timing Diagram for the CS Function of the MCP608 Amplifier
50 nA (typ)
12
Hi-Z
under low humidity
V
IL
t
ON
18.7 A (typ)
18.7 A (typ)
FIGURE 3-7:
MCP606, the A-amplifier of the MCP607 or the
MCP608 in a PC Board Layout
Circuit examples of ring implementations are shown in
Figure 3-8. In Figure 3-8A, B and C, the guard ring is
biased to the common-mode voltage of the amplifier.
This type of guard ring is most effective for applications
where the common-mode voltage of the input stage
changes, such as buffers, non-inverting gain amplifiers
or instrumentation amplifiers.
The strategy shown in Figure 3-8D, biases the com-
mon-mode voltage and guard ring to ground. This type
of guard ring is typically used in precision photo sens-
ing circuits.
Guard Ring
V
t
OFF
IH
Example of Guard Ring for the
-In
Hi-Z
50 nA (typ)
2000 Microchip Technology Inc.
+In
V-